Stabilization of polyethylene



?atented Apr. 13, 1954 2,675,366 STABILIZATION or POLYETHYLENE Joseph C.Pullman, Stamford, Conn, assignor to American Gyanamid Company, NewYork, N. Y., a corporation i Maine No Drawing. Application May 24, 1951,

a Serial No. 228,118

This invention relates to the stabilization of polyethylene againstoxidative deterioration by the incorporation therein 01. small amountsof stabilizing agents. The stabilizing agents or antioxidants used inaccordance with the invention are alkylene bis-(4,6-dialkylpheno1s)which are condensation products of aliphatic aldehydes or ketones with2,4-dialkylphenols.

Polyethylene is a hydrocarbon polymer of considerable commercialimportance as a thermoplastic molding and coating agent. One of its mostimportant advantages is its high'dielectric strength, coupled withrepellent properties against water and water vapor. This causes articlesimpregnated or coated with ethylene polymer to retain their dielectricproperties for considerable periods of time. Polyethylenes having anaverage molecular weight within the range of about 6,000 to about 20,000are therefore of great value as insulators or dielectrics in condensersand other similar electronic equipment.

Unfortunately, however, ethylene polymers are subject to attack byatmospheric oxygen, particularly at elevated temperatures, and when evenslightly oxidized they suffer a reduction in their desirable electricalproperties. The molten polymers also exhibit gelation upon oxidation. Itis a particular object of the present invention to reduce or inhibitthis oxidative deterioration in polyethylene by incorporating thereinsmall but effective quantities or the class of antioxidants referred toabove and hereinafter more particularly described.

I have found that the alkylene bis-(4,6-dialkylphenols) are excellentstabilizers and antioxidants for polyethylene, even when used in verysmall amounts. Appreciable improvements are noted in most cases when aslittle as 0.01% of the antioxidant is used, and effective amounts arewithin the range of 0.05% to 0.5%, based on the weight of thepolyethylene. My present invention therefore consists essentially incontrolling oxidative deterioration in polyethylene by incorporatingtherein stabilizing amounts of alkylene bis-(4,6-dialkylpheno1s) and inthe resulting stabilized polyethylenes.

The alkylene bis-(4,6-dialkylphenols) are prepared by condensing a2,4-dialkylphenol with any suitable aldehyde or ketone. Among the 2,4-dialkylphenols that can be condensed by this procedure and used inpracticing the invention are those obtainable by condensing para-cresolwith aliphatic alcohols or with olefins of 1-8 carbon atoms or more,using suitable Friedel-Crafts catalysts such as aluminum chloride, zincchloride or concentrated sulfuric acid. The conditions of alkylation aredescribed, for example, in Organic Reactions, volume 3 (1946) chapter 1.

Phenols that can be prepared by this method are 2,4-dimethylphenol,2-isopropyl-4-methylphenol, 2-sec.-butyl-4-methylphenol,2-tertbutyll-methylphenol, 2-diisopropyl-4-methylphenol,2-sec.-octyl-4-methylphenol :and 2-triisopropyll-methylphenol, which isprepared commercially by condensing triisopropylene with para-cresol inthe presence of a sulfuric acid catalyst. Any of these or other similarz-alkylated para-cresols may be condensed with formaldehyde under theconditions described above to form 2,2-methylenebis-(4-methyl-6-alkylphenols) for use in practicing the invention, ormay be condensed with other aliphatic aldehydes or ketones.

Typical of the compounds which may be used as polyethylene antioxidantsare 2,2-methylene bis- (4-methyl-6-tert.-butylphenol) 2,.2-methylenebis-(4-methyl-6-tert.-octylphenol), 2,2- methylene bis-(4-methyl-6-tert.-dodecylphenol) 2,2'-'methy1enebis-(4-tert.-butyl-6-methylphen01), 2,2'-methylenebis-(Q-tEKtROCtYI-G-XHBU'IYI phenol), 2,2'-methylenebis-(4-tert.-dodecyl-6- methylphenol), 2,2'methy1ene bis-(4,6-tert.-butylphenol) 2,2'-methylene bis- (4,6tert.-oct-ylphenol), 2,2'-methylenebis-(4,6-tert.-dodecyl phenol), 2,2-methylenebis-(4-methyl-6-amylphenol) 2,2'-methy1ene bis-(4,6-diamylphenol),2,2-methylene bis-(4-tert.butyl-6-tert.-octylphenol), 2,2-methylenebis-(4-tert.-butyl-6 tert.-dodecylphenol) 2,2-methylene bis- (4-tert.octyl-6-tert.dodecylphenol), 2,2'-methylene bis- (4,6 dioctylphenol),2,2 methylene bis (4 methyl-S-dodecylphenol), 2,2'-methylene bis-(4-butyl-6-amylphenol), and 2,2-methylene bis-(4- octyl-fi-dodecylphenol).Other alkylene bis- (dialkylphenols) can be used such as thecorresponding ethylene, propylene and butylene bis- (dialkylphenols),for example 2,2'-ethy1idene bis- (4-methyl-6-tert.-butylphenol):2,2-ethylidene bis-(4-methyl-6-tert.-octylphenol), 2,2- ethylidene bis-(4,6-ditert.-dodecylphenol) 4,4-

isopropylidene his(2,0-=ditert.-hutyly;zheuoll 2.2- lsopropylidenebis-iil-tert.-butyl0-aruylphenol). 2,2-isopropylidehe uol), and the likecompounds.

The invention will be more readily understood by a consideration of thefollowing specific ex amples. Inasmuch as methods for the preparation ofmany or the alkyleue bis-(dfi-dialirylphenols) are not readily availablein the liters. tore, the rst of these examples describe in de tall thepreparation of the antioxidants evaluated, these particular autiozddahtsbeing ore ferred compounds for use in practicum the havention. Examples11-13 inclusive describe the test procedures employed and the resultsobtained.

105 carts (0.54; moi) of 2-methyl-d-tert-butylphenol and 10.5 parts(0.35 mol) of paraformal dehyde are suspended in 125 cc. of heptane and12 parts of concentrated Hill added. The mixture is stirred overnight atroom temperature and then heated to reflux while passing gaseous HClthrough the solution until the water of reaction has distilled overaeeotropically. 60 parts (55% yield) of the product crystallizes :Eromthe cold reaction mixture, melting at ll2-ll8 (1. Recrystallization fromfresh heptaue gives a product melting at i26-l2l C.

Example 2 206 parts (1 mol) of 4,6-ditert-butylpheuol and 16.5 parts(0.55 incl) of paraformaldehyde is dissolved in 200 cc. of heptaue.parts of conceutrated lElCl is added dropwise to the 5011155011 at 3. C.and the rumture is stirred overnight. Solids fail to precipitate fromthe chilled reaction mlxhire at this point, so the reaction mixture isdigested at -60 C. for 2 hours and refluxed at C. for 2.5 hours untilthe Water of reaction has distilled azeotropically. The reaction mixtureis then chilled and 50 parts (24% yield) of the product, melting at138-14l C., is crystallized. Recrystallization from fresh heptaue givesa product melting at Ma i is" 0.

Example 3 103 parts (0.5 mol) of ZA-ditert-butylphenol and 14.5 parts(0.25 mol) of acetone are mixed Without diluent and treated with 21.5parts of concentrated H01 containing 0.25 mol of hydrogen chloride. Thewhole is agitated for 70 hours at room temperature, washed with water,neutralized with gaseous ammonia. and again washed with Water untilneutral. Addition of t he "8.116 is necessary to facilitate Washphenol.10 5 part or Do.

23 parts of containing 50-00 C. for hours. cooled to 20 (3. the solidproduct is collected in a filter. The product is slurried in 500 partsor water containing 20 parts of heptaue 0.25% of Duponol, filtered. anddried. The product Weighs 73 g.; M. P. 125-125 0.; yield 87%. Theaqueous layer is separated l'rom the mother liquor and discarded and 15parts or fresh heptane is added With a fresh charge or 0.5 part of-metliyl-d tert.-= butylpheool, 10 parts of concentrated Hill, and 0.2part of Duponol (3. Processing as above, 50 parts of product isobtained; M. P. 123-l25 (3.; yield Example 5 200 grams (0.91 mol) ofoctylcresol is mixed with 10 grams of concentrated hydrochloric acid.4.2 grams (01. .6 mol) of 36% aqueous formaldehyde solution. is addedthereto and the reaction mixture heated for 1 hour at 55-60 C. Themixture becomes an unstirrahle mass, necessitating the addition of 250cc. of heptane as diluent. after which the mixture is stirred for anaddi tional 2 hours at 55-60 C. The product is then. washed with waterand the heptane-water mix ture removed by azeotropic distillation. 123grams of a substantially colorless viscous product is obtained. 0ncrystallization from heptahe a colorless product is obtained which has amelting point of -418 (3.

Example 0 234 grams (1.0 mol) of ZA-diamylphenol is mixed with 10 gramsof concentrated I-lCl. To the mixtiu'e is added 46 grams (0.55 mol) of36% aqueous formaldehyde solution, with stirring, and the reactionmixture is stirred for e hours at about 50 C. The reacted material isthereafter diluted with 250 cc. of heptane and then thoroughlyWater-washed. The water and heptane are removed by azeotropicdistillation to recover 253 grains of thick, viscous, almost colorlessliquid product.

Example 7 41 grams (0.25 mol) of @methyl-fi-tert-hutyhphenol and 5.5grams (0.125 mol) of acetaldehyde is dissolved in cc. of heptane andtreated with 4.56 grams (0.125 mol) of hydrogen chloride as concentratedH01. The mixture is stirred over night at room temperature and thenheated to reflux to remove water. On removal of the solvent heptahe byvacuum distillation, 35 grams of a mahogany-colored, viscous liquidproduct is obtained. This material crystallizes slowly over a period ofseveral Weeks.

2, bis h-methyl-(i-tert-butyld by the procedure, sube acctaldehyde anequivalent utyraldehyde.

Example 8 -tert.-hutyliethyl-- 350 cc. acetone and strat mus of aqueousiormalde (1.1 mols) or of liuponol C is ad fi 011E111 stirred whilewarmbirth.

l 50"-00 for 3 aiming;

;with water. The water-washed heptano solution is then chilled in an icebath and 158 grams (43%) of the product crystallizes. This material hasa melting point of 108-117 C. n recrystallization from fresh heptane theproduct melts at 123 C.

Example 9 A mixture or 45 grams (0.3 moi) of 2-isopropyl- 4-methylphenol, 13.6 grams (1.65 mols) of aqueous formalin, 30 cc. of amixture of equal parts of water and solvent naphtha. and 1.5 grams ofconcentrated sulfuric acid was heated at 95 C.

Example Fifty parts (0.41 mol) of 2,4-dimethylphencl is mixed with 40parts or concentrated hydrochloric acid containing 14.6 parts (0.40 mol)of hydrogen chloride. 33.2 parts of 36% aqueous formaldehyde solution isadded and the reaction mixture heated on a steam bath for two hours. Theproduct agglomerates into an unstirrable mass, necessitating addition of100 parts of toluene as diluent. The toluene solution of the product iswashed until neutral and chilled in an ice bath. 19 parts (37% yield) ofthe product, melting at 144 C., crystallizes from the solution.Recrystallization from heptane does not charge the melting point of theproduct.

Example 11 The following test procedure was used to evaluate theabove-describedantioxidants in polyethylene. The indicated amounts ofantioxidant were weighed into tall form 200 m1. beakers and a part ofthe polyethylene was added and melted on a hot plate and stirred untilthe antioxidant had dissolved. The remainder of the polyethylene wasthen added and melted with stirring. The beakers were placed in a bathof Woods metal maintained at 240-245 C., which kept the polyethylene at232 C., and heated with stirring for four hours.

The product of Example 4 was tested in this manner at concentrations of0.01%, 0.05% and 0.1%. Another sample of polyethylene containing noantioxidant was heated in the same manner as a control. The polyethyleneused had an average molecular weight of about 12,000.

After 4 hours heating the control sample was extremely viscousandappeared to have gelled. The sample containing 0.01% of antioxidantshowed some increase in viscosity. No increase in the viscosity of theother two samples was observed.

Eazample 12 Samples were prepared as described in the preceding example,using 2,2'-methylene bis-(4- methyl-G-tert.-butylphenol) as theantioxidant, and heated 4 hours at 450 F. while being mechanicallyagitated in the presence of air. The products were cooled and moldedinto disks of about 4 inches diameter and y, inch thick. Elec- 0 atrical dissipation iactors were determined for each disk at a frequencyof 60 cycles. were as follows:

Percent tion Pmwthylen Antioxidant Fa t or None 0. 0005 None 0. 00130.01 0.0000 0.06 0.0000 0.1 0.0003

The action or the antioxidant in retaining the high electricalinsulating properties of the polyethylene is evident from these figures.

Example 13 The procedure of Example 12 was repeated with 0.1 of a numberof other antioxidants. The results are given in the following table:

Product Dissioi Name stion Example actor 2,2'-methylene bis-4-butgl-0-methylphenol)... 0.00023 2,2-methylene bis 4.64ii ut ipheno0.00025 2,2-isoprorylidene 6-d butylphenol).- 0.00022 2,2-methy enebis-(Mnethyl-G-butylpheuol)... 0. 00020 2,2-methy enebis-(4-methyl-6-octyl henoi)-.. 0.00025 2,2'-metb ene bis-4,6-diamylpheno) 0.00023 2,2-ethyi done his- 4-methyl-6-butylphenol)0.00026 2.2'-methylene bisi-etbyl-il-but lphenol?.-.. 0.000302,2-i1nethyleno bis-(4-methyHipropy phe- 0.00027 no 2,2-methylenebis-(4,6 dimethylphenol) 0.00020 2,2-butylidenebis-(kmothyl-B-butylphenol) 0. 00020 From these examples, and from theforegoing discussion, it will be seen that the preferred antioxidantsused in practicing the invention are the 2,2'-alkylenebis-(4,6-dialkylpheno1s) containing an alkylene hydrocarbon radical ofabout 1-4 carbon atoms and aliphatic hydrocarbon substituents on thephenolic groups of chain lengths up to about 12 carbon atoms. Thesecompounds are condensation products of 2 mols of a dialkylphenolcontaining up to 30 carbon higher than those used for antioxidantpurposes; 7

therefore concentrates containing up to 1% to 5% or more of the alkylenebis-(2,4-dialkylphenols) can be prepared and stored and subsequentlyblended with additional quantities of, ethylene polymer to impartantioxidant properties thereto. Thus, for example, a 1% to 5% soiutionof2,2-methylene bis-(4-methyl-6-tert butylphenol) in polyethylene is aconcentrate of substantial commercial value that constitutes a productof commercial value in accordance with the present invention.

What I claim is:

1. A method of controlling oxidative deterioration in polyethylene whichcomprises incorporating therein a small amount, suflicient to reduceoxidation therein, of a 2,2'-methylene bis- (4,6- dialkylphenol) 2. Amethod according to claim 1. in which the bis-phenol is 2,2'-methylenebis-(4-methyl-6- butylphencl).

The results 3. Qzlyathylene stabiliZfl against mum-am we Efizaifmemes9mm im ne file of this patent $5 35,353: g ggmgg 2% 3 6523%???i3j33133fi3fi2'i33? Sp A concentrat; czampose c af aix ifi gs to4 95% by weight of polyethylene and abom 1% mm RGES to 5% of2,2-methy1ene-bis-(4-methy1-6=-butyZ-= Chem. and m. News, v01. 23, No.35, Aug. 28, phenol). m 1950, page 3W3.

1. A METHOD OF CONTROLLING OXIDATIVE DETERIORATION IN POLYETHYLENE WHICHCOMPRISES INCORPORATING THEREIN A SMALL AMOUNT, SUFFICIENT TO REDUCEOXIDATION THEREIN, OF A 2,2''-METHYLENE BIS-(4,6DIALKYLPHENOL).